Low Shrink Unsaturated Polyester Resinous Composition

This invention relates to molding compositions capable of hardening with low shrinkage and based on unsaturated polyesters with high strength, high elongation, and high heat distortion temperatures. In the prior art low shrink unsaturated polyester resins have low strength, low elongation and high heat distortion temperatures. Typical press molding resins have high strength, high elongation, but low heat distortion temperatures. The resin provided by this invention is a low shrink resin, having high strength, high elongation, and high heat distortion temperatures with desirable process or press cycle times.

Conventional polyester molding compositions show considerable polymerization shrinkage which seriously affects the production of moldings with faultless surface qualities. It is known that polyester molding compositions to which certain low shrink additives are added before hardening, can be hardened with low shrinkage. Generally, these low shrink compositions or resins are reinforced with glass fiber or other fiberous reinforcements. Fillers, catalysts, mold releases and pigments are used with such polyester compositions as is known in the art.

In the prior art, typical low shrink additives with unsaturated polyester resins are either styrenated sirups or solids. The styrenated sirups include both acidified and non-acidified poly(methyl methacrylate/ethyl acrylate), poly(vinyl acetate), polystyrene, cellulose acetate butyrate, polycaprolactones, and saturated polyesters, etc. There are usually dissolved at about 30-40% by weight in styrene. Solid additives include polyethylene and poly(vinyl chloride). On solids, the total amount of low shrink additive in the system is about 5 to 20 parts by weight compared to 80 to 95 parts by weight of styrene and unsaturated polyester solids combined. The 100 parts by weight of polyester, styrene and additive are usually referred to as "resin".

Conventional low shrink resins in the prior art have low strength, low elongation and high heat distortion temperatures. Typical press molding resins have high strength, high elongation, but low heat distortion temperatures. It is the object of this invention to have a a low shrink unsaturated polyester resin for fabrication of fibrous reinforced articles by compression, injection or transfer molding with high strength to meet the requirements of a general press resin together with high elongation and high heat distortion temperatures.

Accordingly, the invention provides a low shrink unsaturated polyester resinous composition for molding articles wherein the resin is a product of--

(a) a mixture of 25-35 parts of maleic acid, fumaric acid, their anhydrides and mixtures thereof, 5-25 parts of isophthalic acid, phthalic acid, their anhydrides and mixtures thereof, 10-40 parts propylene glycol, from about 10-30 parts dipropylene glycol and from 5-30 parts propylene glycol terephthalate, said parts being parts by weight based on the total weight of the components;

(b) heating the mixture at about 420.degree. F. with the addition of an inhibitor until a neat resin is formed with an acid number of 5-50; and

(c) diluting the neat resin with 25-100 parts by weight of styrene and/or vinyl toluene.

The resulting polyester resinous composition or neat resin may be compounded into a molding compound when mixed with fillers, fibrous reinforcing agents, pigments, and release agents.

Fillers usually comprise limestone, alumina trihydrate, clay, talc, silicas, wollastonite, alumina, barytes, and gypsum. The two most common are limestone and alumina trihydrate. The latter is essential for good electrical and nonburning properties. The former is preferred when alumina trihydrate is not needed since it is cheaper, flows better, gives better surfaces and stronger molded parts. Silane coupling agents may be utilized with such fillers in accordance with conventional practice.

Reinforcements usually are glass fibers from 150 inch to 2 inches in length. Other reinforcements include textile fibers, polyimide fibers, poly(vinyl alcohol) fibers, boron fibers, graphite fibers and the like as is known in the art.

Catalysts-initiators are usually heat decomposed. Tertiary-butyl perbenzoate is the most common, but by no means the only one. Catalysts are usually peroxides, but can be azo compounds. Benzoyl peroxide, t-butyl peroctoate and azo bis isobutyronitrile. The list is far from being all inclusive and others as are known in the art are catalyst-initiators.

Mold releases include stearate soaps and other compounds as are known in the art. Zinc stearate and calcium stearate are often used.

Processing the molding compound can be done by the usual bulk, sheet or thick molding compound methods. Molding is usually done between 300-1500 psi at 250.degree.-325.degree. F. Compression, transfer and injection techniques may be used. In accordance with this invention, a polyester resinous composition is provided by mixing 5 to 30 parts by weight propylene glycol terephthalate with 25 to 35 parts by weight maleic or fumaric acid or anhydride and 5-25 parts by weight of isophthalic or phthalic acid or anhydride with a glycol or mixture of glycols such as 10 to 40 parts by weight propylene glycol, 10 to 30 parts by weight diporpylene glycol, heating the mixture with an inhibitor until reaching an acid number from 5 to 50. The resulting neat resin is diluted with 25 to 100 parts by weight of styrene or vinyl toluene producing a viscosity of 100-5000 centipoises. The resin can be used as a molding compound when mixed with an initiator-catalyst, filler, reinforcement compound, and a mold release agent.

The invention is illustrated by the following example.

29.9 parts by weight maleic anhydride,

17.5 parts by weight phthalic anhydride,

18.6 parts by weight propylene glycol,

20.0 parts by weight dipropylene glycol, and

14.0 parts by weight propylene glycol terephthalate containing 10 mole per cent excess propylene glycol over that required for complete esterification of the terephthalic acid groups.

are charged into a condensation reactor and heated to 420.degree. F. with the addition of 50 ppm of parabenzoquinone, based on weight of materials charged into the reactor until reaching an acid number of 29. From the original 100 parts, about 92 remain, the remaining 8 parts being converted to water. The resulting neat resin (92 parts) is diluted with 33 parts of styrene, and 100 ppm of hydroquinone based on the total weight of diluted resin composition. The resulting resin has a viscosity of about 800 centipoises. The indicated amounts are in parts by weight.

The above resin is made into a molding compound and molded by adding to 3500 grams of the above resin

19.15 grams of tertiary butyl perbenzoate,

19.25 grams of tertiary butyl peroctoate,

238 grams of zinc stearate,

87.5 grams of fine polyethylene powder,

1750 grams of finely ground limestone, (Vicron 31-6 a product of Pfizer),

4375 grams of finely ground alumina trihydrate (C-330 a product of Alcoa),

88 grams of pigment dispersion, and

140 grams of a magnesium oxide dispersion sold

under the trademark Modifier M.

The above components are mixed on a Cowles type mixer until well stirred and homogeneous. Chopped fiberglass roving of one inch length (Owens-Corning 433 roving), 15% by weight to the above mix, is added and compounded into the mixture in accordance with conventional sheet molding procedures. The resulting polyester glass compound is molded for one minute at 300.degree. F. at 35 mil thickness at 1000 psi, and for 2 minutes at 100 mil thickness at 1000 psi.

The linear shrinkage of the cold molded part to cold mold is 0.4 mil/inch.

Clear castings of the above resin made with 1% benzoyl peroxide to 100 parts by weight resin cured at 70.degree. C. for 1 hour and postcured at 121.degree. C. for 2 hours have the following physical characteristics at 75.degree. F.:

While the invention has been illustrated by the foregoing example, it is understood that various modifications are contemplated and may be made within the scope of the invention, as defined in the following claims.